1. Field of the Invention
The present invention relates to a method of manufacturing a resin molded gear by injection molding. In particular, the present invention relates to a technology in which a high-precision resin molded gear, which includes a web and a rim and includes a tooth portion arranged at an outer peripheral portion of a molded product, is manufactured by providing, in an injection molding process, temperature ranges for resin temperature of predetermined regions of the gear so as to suppress deformation of the tooth portion, dimensional fluctuation, and increase in shrinkage.
2. Description of the Related Art
Resin molded gears are mounted as power transmission components to a wide variety of mechanical products including office automation (OA) equipment such as a copying machine and a printer, consumable supplies such as an ink cartridge, and small precision equipment such as a digital camera and a video camera.
Conventionally, as the resin molded gear serving as the power transmission component, a spur gear is widely used.
In a case where an improvement in noise reduction performance and contact ratio is highly demanded, a helical gear is used. In recent years, along with an improvement in functionality and quality of mechanical products, there has also been a demand for higher-precision gears, and hence standards for roundness, concentricity, and the like as well as the standards for the helix grading (JIS B 1702) and the contact error (JGMA 116-02) are set to narrower standard ranges in many cases.
As a resin to be used for molding such a gear as described above, polyacetal, polyethylene, nylon, polybutylene terephthalate, polyethylene terephthalate, and polypropylene are taken as examples thereof.
However, any of the resins described above undergoes volumetric shrinkage when the resin changes from a molten state to a solidified state, and further, deformation and sink of a molded product occur depending on conditions such as pressure dwell setting, mold temperature setting, and resin temperature setting, which raises a problem in that gear precision cannot be satisfied.
For example, in a case where polyacetal is used as the resin for molding, a shrinkage percentage thereof is high because the resin is a crystalline resin, and resin temperature is not easily decreased due to latent heat generated at the time of crystallization, which raises a problem in that the molded product is likely to deform. To address this problem, as a conventional technology, there are disclosed a technique of devising a cooling method for a mold to enhance cooling performance, and a technique of pressurizing a surface of the molded product in a cooling process to suppress the deformation and shrinkage thereof.
Further, there is disclosed a technique of setting a partial region of the gear to a predetermined shape or a predetermined thickness so as to suppress the shrinkage.
For example, in Japanese Patent Application Laid-Open No. 2007-130902, a depression concentric with a tooth portion of the gear is provided in a die on a movable mold side, and a projection having a shape conforming to the shape of the depression on the movable side is provided in a die on a stationary mold side so as to be mated with the depression when the mold is closed.
Japanese Patent Application Laid-Open No. 2007-130902 discloses a technology in which a cooling medium channel concentric with the tooth portion is arranged inside the projection, to thereby enhance the cooling performance for the tooth portion of the gear.
Further, Japanese Patent Application Laid-Open No. 2002-235835 discloses a technology in which a step of pressurizing part of a gear-shaped portion with use of a pressurization mechanism mounted to the mold is provided in an injection molding process in addition to a pressure dwelling step, to thereby provide a high-precision resin molded gear having an improved shape and dimensional accuracy.
Further, Japanese Patent Application Laid-Open No. H11-13861 discloses a technology in which, in a resin molded gear including a web and a rim and including a tooth portion arranged at an outer peripheral portion of a molded product, thicknesses of the rim and the web are each set at a ratio in a given range relative to a pitch circular thickness of the tooth, to thereby obtain a high-precision gear.
When the gear is molded, shrinkage inevitably occurs, and it has been known that the shrinkage affects the gear precision depending on a shrinkage amount and a shrinkage tendency.
Further, it has also been known that deformation and dimensional fluctuation as well as the shrinkage affect the gear precision. There are provided several measures for suppressing the shrinkage in the injection molding. Among others, the following five factors are most effective to be changed: (1) thickness of the molded product, (2) gate dimension, (3) injection pressure, (4) screw advancing time period, and (5) mold temperature.
However, the contents of the items (1) and (2) may be hard to change depending on intended use of the molded product.
Therefore, the measures to be generally taken for reducing the shrinkage mainly include changing of the molding conditions of the items (3) to (5).
Hereinafter, a relationship between the shrinkage percentage and the injection pressure, the screw advancing time period, and the mold temperature is specifically described.
Injection pressure: As the injection pressure becomes higher, the shrinkage percentage becomes lower.
However, even when a high injection pressure is applied, the pressure to be transmitted into a cavity varies depending on the degree of a local pressure loss. Therefore, the pressure loss becomes most significant in the vicinity of a final loading section located at the largest resin flowing distance. Accordingly, in this region, the shrinkage amount increases and also the dimensional fluctuation or the like is likely to occur.
Screw advancing time period: The screw advancing time period refers to a time period in which the resin inside the cavity is continuously compressed by the injection pressure, that is, a time period from the start of advancing the screw or the plunger to the start of retreating the screw or the plunger.
When the screw advancing time period reaches to a gate solidification time point, the shrinkage percentage becomes lowest, but when the screw advancing time period falls short of the gate solidification time point, the shrinkage percentage increases.
Mold temperature: As the mold temperature becomes lower, the shrinkage percentage becomes lower.
However, as the mold temperature becomes lower, the pressure loss becomes more significant in the process in which the resin flows, with the result that the fluctuation occurs in the pressure to be transmitted into the cavity.
Further, along with deterioration of flowability, the surface property may deteriorate.
That is, it is found that the shrinkage phenomenon depends significantly on the pressure state and the mold temperature state inside the cavity. Further, those two states significantly affect the deformation and the dimensional fluctuation as well as the shrinkage.
Next, the effects of the pressure state and the mold temperature state inside the cavity are described in detail by taking a gear of FIG. 2 as an example.
The gear includes a rim 11 formed into a cylindrical shape, teeth 12 formed along an outer peripheral surface of the rim in an outward direction from a center axis 15 of the cylindrical rim, a web 13, which joins to an inner peripheral surface of the rim and extends in a direction toward the center axis to have a flat disc shape, a boss 14, which joins to the web and is formed on a core portion located at the center axis, and a gate 16.
As the pressure to be applied inside the cavity becomes higher, the shrinkage percentage becomes lower, and the dimensional fluctuation also becomes smaller. Thus, the higher injection pressure is more effective.
However, after the resin is loaded into the cavity, when a cylinder advances in a pressurization step to progress solidification of the resin at the boss or web portion, the pressurization state attenuates at the rim and the tooth portion, which are located at the largest distance in the pressure transmission path. As a result, sufficient pressurization performance cannot be maintained until the gate is solidified, which raises the problems of dimensional fluctuation and deformation occurring in the rim and the tooth portion.
In order to suppress the attenuation of the pressurization state, the mold temperature is increased as a measure therefor. That is, there is used a method of delaying the transition to the solidified state at the boss and web portion due to a high mold temperature, to thereby maintain the pressurization state until the gate is solidified.
However, as described above, when the mold temperature is high, the shrinkage percentage of the molded product increases, which raises a problem of reduction in gear precision.
Japanese Patent Application Laid-Open No. 2007-130902 discloses the technique of enhancing the cooling performance for the tooth portion by the fittable die having the medium channel, but the pressurization state of the gear is not taken into consideration, and accordingly it is difficult to solve the problems such as the dimensional fluctuation and the deformation. Further, such a configuration potentially raises problems with durability of the mold and manufacture of the mold.
Further, Japanese Patent Application Laid-Open No. 2002-235835 discloses the technique of enhancing the gear precision with use of the pressurization mechanism, but the mold structure becomes complicated so that the manufacture of the mold becomes difficult, and further, there is a problem in that the number of steps in the molding process increases.
Further, Japanese Patent Application Laid-Open No. H11-13861 discloses the technique of suppressing the sink and the deformation by changing the gear shape. However, there is a problem in that the strength and the durability of the gear itself remarkably change due to the changing of the gear shape such as the thickness thereof.